In the manufacture of an electronic device such as a semiconductor device, a plasma process, for example, a plasma etching process is performed on a processing target object. As one kind of plasma processing apparatuses used to perform the plasma process, there is known a capacitively coupled plasma processing apparatus.
The capacitively coupled plasma processing apparatus generally includes a processing vessel, an upper electrode, a lower electrode, a first high frequency power supply, a first matching device, a second high frequency power supply and a second matching device. The upper electrode and the lower electrode are disposed to be substantially parallel to each other with a space therebetween within the processing vessel. The first high frequency power supply generates a first high frequency power for plasma generation. The first high frequency power is supplied to either one of the upper electrode and the lower electrode. Further, the second high frequency power supply generates a second high frequency power for ion attraction. The second high frequency power is supplied to the lower electrode. Further, in the capacitively coupled plasma processing apparatus, a variable reactance element of the first matching device is adjusted to match an output impedance of the first high frequency power supply and an impedance at a load side thereof. Further, a variable reactance element of the second matching device is adjusted to match an output impedance of the second high frequency power supply and an impedance at a load side thereof. In this capacitively coupled plasma processing apparatus, a gas supplied into the processing vessel is dissociated by a high frequency electric field which is generated between the upper electrode and the lower electrode, so that the processing target object is processed by active species such as ions or radicals.
Further, in the capacitively coupled plasma processing apparatus, a modulated high frequency power, whose power is pulse-modulated, may be supplied from at least one of the first high frequency power supply and the second high frequency power supply. That is, there may be used a modulated high frequency power in which a first period and a second period are repeated alternately and a power in the second period is lower than a power in the first period. By way of example, a modulated high frequency power for plasma generation is used to suppress an increase of an electron temperature or to suppress a charging damage of the processing target object, and a modulated high frequency power for ion attraction is used to suppress a microloading effect.
If such a modulated high frequency power is used, an impedance at the load side, particularly, an impedance of plasma is varied in synchronization with a cycle of the pulse modulation. As a result, a reflection wave heading toward a corresponding high frequency power supply is increased at the beginning of the first period. To reduce this reflection wave, the impedance at the load side needs to be matched with an output impedance of the high frequency power supply, i.e., a matching point at a high speed. Since, however, a variable reactance element of a matching device is generally configured to adjust a reactance thereof by using a mechanical device such as a motor, it is difficult for the matching device to perform impedance matching at a high speed.
In this regard, there is employed a technique in which a high frequency power output from a high frequency power supply in a period including the starting of the first period is set to have a preset frequency higher than a frequency of the high frequency power in a steady period of the corresponding first period. This technique is described in, for example, Patent Document 1.
Patent Document 1: Japanese Patent No. 3,122,618
In the technique described in Patent Document 1, the preset frequency is a fixed frequency. Accordingly, the frequency of the high frequency power cannot be adaptively changed according to a variation of the impedance at the load side. As a result, there is a limit in suppressing the reflection wave. Further, a high modulation frequency is required for the pulse modulation of the power of the high frequency power. Thus, it is required to control the frequency of the high frequency power adaptively and rapidly according to a change in the impedance at the load side of the high frequency power supply.